1. Field of the Invention
The present invention relates generally to internal combustion engines and, more particularly to, a low friction valve train for an internal combustion engine.
2. Description of the Related Art
It is known to construct valve trains for opening and closing valves in engines such as internal combustion engines. Such a valve train may be a direct acting hydraulic bucket tappet valve train for an overhead cam type internal combustion engine. Generally, the valve train includes a tappet which contacts a cam on a cam shaft which is used to translate rotational motion of the cam shaft into axial motion of the valve. The valve is closed by a valve spring which biases the valve in a closed position.
The valve train includes a hydraulic lash adjuster which compensates for a change in valve length due to thermal expansion caused by temperature changes as well as valve seat wear. This type of valve train is a high pressure system which, through hydraulic pressure generated by the lubrication system, keeps the valve lifter in proper contact with the cam to perform the valve opening/closing function. The constant hydraulic pressure continuously applied to the valve to maintain proper contact with the cam, in addition to the forces induced by the cam, results in increased friction losses and significant wear to the components of the valve train.
However, the hydraulic pressure is expected to provide hydrodynamic film lubrication between a journal of the cam and bearing surfaces of the cam shaft, and the tappet surface and the cam surfaces. Because of high unit loads, the valve train operates in a predominately boundary/mixed lubrication regime, particularly in the 750-2000 engine speed range. This speed range represents more than 80% of the driving cycle for passenger vehicle operation. Because the operation is in the predominantly boundary/mixed lubrication regime, the contacting components are subject to significant wear, as much as 30 to 150 microns on the cam during the life of the engine.
Additionally, engine speed is limited by the incidence of "valve toss" which is due to the reciprocating mass of the valve train. Reducing the valve train mass decreases the forces due to inertia and, as a result, permits higher engine operating speeds which, in turn, result in greater engine output. Further, reducing the friction between the moving components significantly reduces the wear and eliminates the need for a heavy, complex and expensive hydraulic system and enables the engine to operate at normal hydraulic pressures without the friction losses and corresponding wear encountered in standard hydraulic systems. The reduction in friction, in turn, results in fuel economy improvement and the reduction in wear improves component durability and, as a consequence, engine life. Thus, there is a need in the art to reduce the mass of the valve train and friction between moving components of the valve train.